Polymer materials are being increasingly used in various engineering applications due to their multiple advantages over traditional metallic materials. Their light weight, high plasticity, corrosion resistance and good insulation are being exploited for applications in the basic support components of instrument board, municipal water supply pipelines network, and insulation parts in the electrical power system, etc. However, polymer materials are always exposed to a variety of degrading influences during normal services, such as extreme temperature, light rays, and oxidation, failures caused by the degradation usually occur. Take the composite insulators which are widely used in the power supply system, for example; the external insulation part is made of high-temperature vulcanization silicone rubber (HTVSR), which is a typical polymer material. To ensure the self-cleaning ability of the composite insulators under contaminated conditions, the umbrella skirt structure is designed to be an inclined plane with a certain angle, whose thickness decreases gradually from nearby of the fiberglass reinforced resin rod to the edge of the umbrella skirt. Under the integrated effects of high voltage surge and environmental factors over extended periods of service, irreversible aging of the HTVSR material occurs. Aging of the composite insulators will lead to an electrical property degradation of the external insulation, which is a serious potential threat to the safe and stable running of the high voltage power grid. Therefore, it is necessary to develop non-destructive testing (NDT) methods for aging damage detection of polymer materials. Especially for the thickness gradually changed structure aging damage detection has significant engineering application values.
Various testing techniques such as the visual inspection, the tensile test method, and infrared spectroscopic analysis are used for the characterization of aging damage degrees of polymer materials. Generally, it is difficult to evaluate the material performance based on the appearance of materials, mechanical properties and its molecular structure. In addition, the above-mentioned methods can not directly characterize the dielectric and insulation properties of polymer materials. Capacitive proximity sensors are newly developed sensing technique based on the fringing effect of the electric field. Capacitance variation caused by the relative dielectric permittivity changed is closely related to the electrical properties of the polymer materials and can be utilized to detect and evaluate the electrical performance of dielectric structures with low conductivity. Compared with the traditional parallel-plate capacitor, capacitive proximity sensors are widely used for a variety of parameters measurement attribute to their features: high sensitivity, non-invasive, and the planar structure that is particularly useful when the access to a test specimen is limited to the only one side. Therefore, they have been widely used in many fields, such as material property monitoring, damage detecting, humidity sensing, electrical insulation properties sensing, chemical sensing, and bio-sensing, etc.
The capacitive proximity sensor is mainly composed of a driving electrode, a sensing electrode, a shielding layer, and a substrate. Existing research indicates that capacitive sensor performances such as signal strength, penetration depth, sensitivity, and noise-to-signal ratio are greatly influenced by the patterns and parameters of the electrodes, which affect the detecting capability of capacitive proximity sensors.
Many efforts have been devoted to improving the performance of capacitive proximity sensors. Several sensor patterns including square-shaped, maze, spiral and comb patterns were investigated by LI (Design principles for multichannel fringing electric field sensors [J]. Sensors Journal, IEEE, 2006, 6(2): 434-440), and it was demonstrated that complex sensor patterns could increase the effective electrode area and then improve sensor signal and sensitivity. A capacitive sensor of interdigitated electrode structure with increased height was fabricated for humidity measurement (Kim J H, Moon B M, Hong S M. Capacitive humidity sensors based on a newly designed interdigitated electrode structure [J]. Microsystem technologies, 2012, 18(1): 31-35). Compared to the traditional interdigitated electrode sensor, the proposed sensor showed higher sensitivity owe to the horizontal electric field lines confined in the polyimide sensing layer. To improve signal strength and sensitivity, Rivadeneyra et al. (Rivadeneyra A, Fernandez-Salmeron J, Banqueri J, et al. A novel electrode structure compared with interdigitated electrodes as capacitive sensor [J]. Sensors and Actuators B: Chemical, 2014, 204: 552-560) designed a serpentine structure which is a combination of meandering and interdigitated electrodes. Compared with the traditional interdigitated sensor, the signal strength of the sensor increased by 28%.
Existed research works indicated that there is a restriction relationship among the signal strength, sensor sensitivity and penetration depth for capacitive sensors; so in practical applications, a tradeoff of the sensors' properties need to be considered in the actual sensor designing process. Anything else, the structures designing and parameters optimizing of the capacitive sensor existed in previous research works are all based on the equal thickness structure. Thus the spacing between two adjacent electrodes of conventional interdigitated electrode (IDE) structure sensors is equal. For conventional IDE sensors, the penetration depths of the electric field between two adjacent electrodes are approximately equal, and the sensor is not suitable for the detection of the thickness gradually changed the structure. Aiming at the dielectric property inspection for a thickness decreases gradually structure such as umbrella skirt on the composite insulators, it is intended to design an improved IDE structure sensor which spacing between two adjacent electrodes are not equal, and the electric field of the newly designed sensor is confined within the cross section of the material under test (MUT). The optimal design of the interdigitated capacitive sensor with variable spacing electrode structure is rarely reported.